The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuits is known in the art. Such technology is of increasing interest in the production of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Thick-film multilayer structures based on copper conductors typically are comprised of at least two patterned layers of copper conductor separated by a dielectric layer. The patterned conductor layers are connected by copper deposited in vias in the dielectric layer. Such structures are formed by multiple deposition and firing of layers of copper and dielectric inks.
Such multilayer circuit structures utilizing copper as the conductor metal have a number of problems. The most common is failure caused by the development of electrical shorts due to interactions between flux materials of the copper conductor ink and the dielectric layer which take place during the multiple firings necessary to fabricate a multilayer integrated circuit. The responsible materials present in conventional copper conductor inks include copper oxide which forms upon exposure of the ink to air or an oxidizing atmosphere and flux materials such as lead oxide and bismuth oxide. These materials will penetrate a porous dielectric material, particularly if it contains large modifier ions such as lead, barium and bismuth. The penetration of such materials is enhanced by the fact that the multiple firing steps are conventionally carried out at temperatures well above the temperature at which copper oxide forms an eutectic mixture with lead or bismuth oxides.
A second problem common to both dielectric and copper conductor inks is the entrapment of gaseous materials formed during repeated firings as a result of incomplete removal of the carbonaceous residue of the organic vehicle present in conventional ink formulations. The escaping gaseous material can cause blistering and peeling of the thick films formed from the inks during subsequent firings and is also responsible for the porosity of dielectric films.
An approach to reducing the incidence of electrical shorts from these problems is to formulate dielectric inks which form thick films having reduced porosity.
A second approach is to treat functional inks, e.g. dielectric and copper conductor inks, with an oxidizing or reducing plasma prior to firing as disclosed by Prabhu et al. in U.S. Pat. No. 4,619,836, issued Oct. 28, 1986. The plasma treatment removes the carbonaceous residue of the organic vehicle present in conventional ink formulations.
A third possibility is the formulation of improved copper conductor inks. Such improved inks are provided in accordance with this invention.